ValkeyBatch.PreExecCheck() was a virtual hook that existed solely to
support transaction preconditions. Batches have no concept of
preconditions, so this was transaction-specific logic leaking into the
base class.

Additionally, ValkeyTransaction.PreExecCheck() called
ExecuteImpl().GetAwaiter().GetResult() — the last remaining
sync-over-async call in the codebase — which risks deadlocks in
environments with a synchronization context (e.g. ASP.NET).

Changes:
- Remove virtual PreExecCheck() from ValkeyBatch and the
  if (PreExecCheck()) guard from ExecuteImpl(). ExecuteImpl() now
  unconditionally executes the batch, which is the correct behaviour
  for ValkeyBatch (conditions are a transaction concern only).
- Move condition evaluation into ValkeyTransaction.ExecuteAsync()
  directly, using await instead of .GetAwaiter().GetResult().
- Short-circuit early (set _tcs result to null and return false) as
  soon as any condition fails, avoiding unnecessary work.
- Rename the inner batch variable from b to conditionBatch for clarity.

Fixes valkey-io#266

Signed-off-by: currantw <taylor.curran@improving.com>